Recently, a request has been increased in order to develop excellent ultraviolet ray and thermic ray shut-off glass which is able to prevent interiors from being deteriorated and getting sunburnt, to lessen a sense of heat, and to decrease the load of air conditioner, by shutting off ultraviolet rays and thermic rays of sunlight which flows into a compartment of automobile, etc., rooms of house, office, etc. In order to achieve such a request, ultraviolet ray absorbing and thermic ray reflecting glass having a high refractive index layer and a low refractive index layer, which have a ultraviolet ray absorbing power, alternately laminated on the surface of glass, and ultraviolet ray absorbing and thermic ray reflecting glass which allows visible light beams to pass through and is able to selectively reflect the thermic rays have been developed.
In order to use the abovementioned ultraviolet ray absorbing and thermic reflecting glass as a window shield for automobile for which the lower limit of visible light beam transmittance is defined to be 70%, it is necessary to increase the reflection index of only the thermic ray area without decreasing the transmittance of visible light beams. For this reason, there are some methods for high refractive index layer and low refractive index layer, one of which is a method of increasing the number of interfaces (number of times of lamination), and the other of which is a method of increasing the difference between the refractive index of the high refractive index layer and that of the low refractive index layer.
For example, Japanese laid-open patent publication No. 345488 of 1994 discloses thermic ray reflecting glass composed of two layers of high refractive index layer and low refractive index layer, which is covered with a thermic ray reflecting layer. The low refractive index layer described therein has a refractive index which is the middle between the glass substrate and the high refractive index layer. It can not be said that the thermic ray reflecting performance is sufficient since the thermic ray reflecting glass has two layers and the low refractive index layer thereof has a middle refractive index.
Furthermore, Japanese laid-open patent publication No. 177204 of 1992 discloses a ultraviolet ray and infrared ray cut filter, and Japanese laid-open patent publication No. 104544 of 1996 discloses thermic ray reflecting and ultraviolet ray absorbing glass and a method for producing the same. High refractive index film used for thermic ray reflecting glass disclosed in these publications employs a ultraviolet ray absorbing film mainly composed of cerium oxide or cerium oxide and titanium oxide as described in a literature [Akio Makishima, et al., J. Am. Ceram. Soc., 69[6]C-127-C-129 (1986)]. A film is composed so as to have a combination ratio of cerium oxide and titanium oxide so that the ultraviolet ray absorbing power can be displayed.
Furthermore, Japanese laid-open patent publication No. 239244 of 1996 describes ultraviolet ray absorbing glass which is composed of three layers of high refractive index layer, low refractive index layer and high refractive index layer, each layer having an optical film thickness from 170 nm to 400 nm, wherein it is disclosed that one layer of the two high refractive index layers consisting of titanium oxide is caused to contain cerium oxide, preferably to have a ratio by weight of cerium oxide to titanium oxide, which is 0.1 to 5.0.
Furthermore, Japanese laid-open patent publication No. 281023 of 1995 describes a high refractive index film containing titanium oxide and bismuth oxide and an incandescent lamp bulb having infrared ray reflecting film using the same.
However, in the abovementioned ultraviolet ray and infrared ray cut filter (Japanese laid-open patent publication No. 177204 of 1992), thermic ray reflecting and ultraviolet ray absorbing glass (Japanese laid-open patent publication No. 104544 of 1996), and ultraviolet ray absorbing glass (Japanese laid-open patent publication No. 239244 of 1996), the thermic ray reflecting performance was not fully displayed to the maximum. Furthermore, with a high refractive index film employed for the abovementioned incandescent lamp bulb having an infrared ray reflecting film (Japanese laid-open patent publication No. 281023 of 1995), sufficient ultraviolet ray absorbing power could not be displayed.
The present invention was developed in order to solve these conventional shortcomings and problems, and it is therefore an object of the invention to provide an optical thin film having a comparatively high refractive index in order to heighten the thermic ray reflection selecting performance and having a ultraviolet ray absorbing performance and ultraviolet ray absorbing and thermic ray reflecting glass using the same.